Efficient computing procedures have been developed to simulate the behavior in the ultracentrifuge of solutes involved in rapid mixed association equilibria: nA plus mB yields reversibly AnBm. The technique is able to deal with systems containing any number of agregated species, provided noen is present in which both n and m are greater than two. Combination of this method with the finite-element procedure of Claverie will add the capability to treat systems involved in slow reequilibration and solutes that diffuse too rapidly to allow neglect of reflected diffusion at the meniscus. The methods can be recast to treat other transport experiments: chromatography, electrophoresis, and isoelectric focussing. Comparison of real transport experiments with simulations is being used to examine several associating systems. The dissociation of hemoglobin from the bullfrog tadpole is being studied as a function of pH. Interactions of the small histones from chicken erythrocytes will be considered at a range of pH and salt concentration over which H3-H4 tetramers and H2A-H2B dimers are believed to associate to yield nucleosome cores. Interactions between nitrogenase components from various nitrogen-fixing bacteria will be compared.